One example of such a system, whereby mechanical energy may be converted into electrical energy, is a triboelectric energy generation system. The triboelectric effect (also known as triboelectric charging) is a contact-induced electrification in which a material becomes electrically charged after it is contacted with a different material through friction. Triboelectric generation is based on converting mechanical energy into electrical energy through methods which couple the triboelectric effect with electrostatic induction. It has been proposed to make use of triboelectric generation to power mobile wearable devices such as sensors and smartphones by capturing the otherwise wasted mechanical energy from such sources as walking, random body motions, the wind blowing, vibration or ocean waves. See, for example: Wang, Sihong, Long Lin, and Zhong Lin Wang, Triboelectric nanogenerators as self-powered active sensors, Nano Energy 11 (2015): 436-462).
The triboelectric effect is based on a series that ranks various materials according to their tendency to gain electrons (become negatively charged) or lose electrons (become positively charged). This series is for example disclosed in A. F. Diaz and R. M. Felix-Navarro, A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties, Journal of Electrostatics 62 (2004) 277-290. The best combinations of materials to create static electricity are one from the positive charge list and one from the negative charge list (e.g. PTFE against copper, or FEP against aluminium). Rubbing glass with fur, or a comb through the hair are well-known examples from everyday life of triboelectricity.
In its simplest form, a triboelectric generator thus uses two sheets of dissimilar materials, one an electron donor, the other an electron acceptor. One or more of the materials can be an insulator. Other possible materials might include semiconductor materials, for example silicon comprising a native oxide layer. When the materials are brought into contact, electrons are exchanged from one material to the other. This is simply the triboelectric effect. If the sheets are then separated, each sheet holds an electrical charge (of differing polarity), isolated by the gap between them, and an electric potential is built up. If an electrical load is connected between electrodes deposited/placed at the backside of the two material surfaces, any further displacement of the sheets, either laterally or perpendicularly, will induce in response a current flow between the two electrodes. This is simply an example of electrostatic induction. As the distance between the respective charge centers of the two plates is increased, so the attractive electric field between the two, across the gap, weakens, resulting in an increased potential difference between the two outer electrodes, as electrical attraction of charge via the load begins to overcome the electrostatic attractive force across the gap.
In this way, triboelectric generators convert mechanical energy into electrical energy through a coupling between two main physical mechanisms: contact electrification (tribo-charging) and electrostatic induction.
By cyclically increasing and decreasing the mutual separation between the charge centers of the plates, so current can be induced to flow back and forth between the plates in response, thereby generating an alternating current across the load. Triboelectric generator devices thus can be considered as charge pumps.
The power output can be increased by applying micron-scale patterns to the polymer sheets. The patterning effectively increases the contact area and thereby increases the effectiveness of the charge transfer.
Recently, an emerging material technology for power generation (energy harvesting) and power conversion has been developed which makes use of this effect, as disclosed in Wang, Z. L., “Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors.” ACS nano 7.11 (2013): 9533-9557. Based on this effect several device configurations have been developed of so-called triboelectric generators (“TEG”).
Since their first reporting in 2012, the output power density of TEGs has been greatly improved. The volume power density may reach more than 400 kilowatts per cubic meter, and an efficiency of ˜60% has been demonstrated (ibid.). In addition to high output performance, TEG technology carries numerous other advantages, such as low production cost, high reliability and robustness, and low environmental impact.
The TEG may be used as an electrical power generator, i.e. energy harvesting from, for example, vibration, wind, water or random body motions or even conversion of mechanically available power into electricity. The generated voltage is a power signal.
TEGs may broadly be divided into four main operational classes.
A first mode of operation is a vertical contact-separation mode, in which two or more plates are cyclically brought into or out of contact by an applied force. This may be used in shoes, for example, where the pressure exerted by a user as they step is utilized to bring the plates into contact. One example of such a device has been described in the article “Integrated Multilayered Triboelectric Nanogenerator for Harvesting Biomechanical Energy from Human Motions” of Peng Bai et. al. in ACS Nano 2013 7(4), pp. 3713-3719. Here, the device comprises a multiple layer structure formed on a zigzag shaped substrate. The device operates based on surface charge transfer due to contact electrification. When a pressure is applied to the structure, the zigzag shape is compressed to create contact between the different layers, and the contact is released when the pressure is released. The energy harvested might for example be used for charging of mobile portable devices.
A second mode of operation is a linear sliding mode, wherein plates are induced to slide laterally with respect to one another in order to change the area of overlap between them. A potential difference is induced across the plates, having an instantaneous magnitude in proportion to the rate of change of the total overlapping area. By repeatedly bringing plates into and out of mutual overlap with one another, an alternating current may be established across a load connected between the plates.
A design which enables energy to be harvested from sliding motions is disclosed in the article “Freestanding Triboelectric-Layer-Based Nanogenerators for Harvesting Energy from a Moving Object of Human Motion in Contact and Non-Contact Modes” in Adv. Mater. 2014, 26, 2818-2824. A freestanding movable layer slides between a pair of static electrodes. The movable layer may be arranged not to make contact with the static electrodes (i.e. at small spacing above the static electrodes) or it may make sliding contact.
A third mode of operation is a single electrode mode in which one surface is for example grounded—for example, a floor road—and a load is connected between this first surface and ground (see for example Yang, Ya, et al. “Single-electrode-based sliding triboelectric nanogenerator for self-powered displacement vector sensor system,” ACS nano 7.8 (2013): 7342-7351.). The second surface—not electrically connected to the first—is brought into contact with the first surface and tribocharges it. As the second surface is then moved away from the first, the excess charge in the first surface is driven to ground, providing a current across the load. Hence only a single electrode (on a single layer) is used in this mode of operation to provide an output current.
A fourth mode of operation is a freestanding tribo electric layer mode, which is designed for harvesting energy from an arbitrary moving object to which no electrical connections are made. This object may be a passing car, passing train, or a shoe, for example (Again, see “Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors”, ACS nano 7.11 (2013): 9533-9557).
One particular subset of linear sliding mode TEGs which have been developed are rotational disc TEGs which can be operated in both a contact (i.e. continuous tribocharging and electrostatic induction) or a non-contact mode (i.e. only electrostatic induction after initial contact electrification). Rotational disc TEGs typically consist of at least one rotor and one stator each formed as a set of spaced circle sectors (segments). The sectors overlap and then separate as the two discs rotate relative to each other. As described above, a current may be induced between two laterally sliding—oppositely charged—layers, with a magnitude in proportion to the rate of change of the area of overlap. As each consecutively spaced sector of the rotor comes into and then out of overlap with a given stator sector, so a current is induced between the two sector plates, initially in a first direction, as the plates increase in overlap, and then in the opposite direction as the plates decrease in overlap.
The limitations of early versions of segmentally structured disc TEGs (Long Lin et al., Segmentally Structured disc Triboelectric Nanogenerator for Harvesting Rotational Mechanical Energy, Nano Lett., 2013, 13 (6), pp. 2916-2923) were that the rotational and stationary triboelectric layers require deposition of metal electrodes and connection with electrical leads, leading to inconvenient operation of the rotational part. Furthermore intimate contact is mandatory to achieve efficient electricity generation, which results in possible material wear, wear particles, instability of output, and generally limited lifetime of the TEG.
A disc TEG with both groups of patterned electrodes attached onto a stationary disc, together with a freestanding triboelectric layer on a rotational disc can resolve these issues, as disclosed in Long Lin et al., Noncontact Free-Rotating disc Triboelectric Nanogenerator as a Sustainable Energy Harvester and Self-Powered Mechanical Sensor. ACS Appl. Mater. Interfaces, 2014, 6 (4), pp. 3031-3038.
With such a structure, there is no necessity for electrode deposition or electrical connection for the rotational part, which dramatically improves the operating facility of the energy harvester.
Although the TEG shows promise, it has challenges when the output power of the TEG needs to be converted to voltage and current levels for practical applications that include electronics such as micro controllers. At such low power levels produced by TEG, it is key to have an efficient power conversion stage.
A power conversion stage is required to convert the TEG voltage in the range of a few hundreds of volts to a low voltage such as below 10V. Converting such low power levels by means of a switched-mode power supply (SMPS) is not really practical as a high inductance value is required. Moreover, the supply current required for the controller is usually in the same range as is generated by the TEG. In this case, an external power supply would typically be needed in order to power the SMPS converter. This situation is, as expected, not desirable since the power consumption of the converter would be of the same order of magnitude as the power generated by the TEG, which would lead to a low power conversion efficiency of the system.
Another disadvantage of using SMPS converters in TEG applications is the difficulty in generating the driving signals of these converters. The driving signals of a SMPS converter should for example be synchronized to the generated output signal of its TEG. Since TEGs can generate fast varying signals over time, this synchronization is not straightforward.
Switched capacitor converters can for example be implemented as power converters. Although such converters do not require any inductance, their driving signals still need to be correctly synchronized with the signal generated by the TEGs. Such synchronization—which is also of importance for other aspects such as feedback for controlling of power output or for sensing functionality—is not straightforward.